cherry pick pingcap#3153 to release-2.1

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

faq/tidb-faq.md

@@ -106,7 +106,7 @@ As a standalone database, MySQL can only implement across-database transactions
 The display content of TiDB `show processlist` is almost the same as that of MySQL `show processlist`. TiDB `show processlist` does not display the system process ID. The ID that it displays is the current session ID. The differences between TiDB `show processlist` and MySQL `show processlist` are as follows:
 
 - As TiDB is a distributed database, the `tidb-server` instance is a stateless engine for parsing and executing the SQL statements (for details, see [TiDB architecture](/architecture.md)). `show processlist` displays the session list executed in the `tidb-server` instance that the user logs in to from the MySQL client, not the list of all the sessions running in the cluster. But MySQL is a standalone database and its `show processlist` displays all the SQL statements executed in MySQL.
-- TiDB `show processlist` displays the estimated memory usage (unit: Byte) of the current session, which is not displayed in MySQL `show processlist`.
+- The `State` column in TiDB is not continually updated during query execution. As TiDB supports parallel query, each statement may be in multiple _states_ at once, and thus it is difficult to simplify to a single value.
 
 #### How to modify the user password and privilege?
 
@@ -543,7 +543,7 @@ In the communication process between the TiDB server and the TiKV server, the `S
 
 #### What's the maximum number of concurrent connections that TiDB supports?
 
-The current TiDB version has no limit for the maximum number of concurrent connections. If too large concurrency leads to an increase of response time, you can increase the capacity by adding TiDB nodes.
+By default, there is no limit on the maximum number of connections per TiDB server. A limit may be enforced by setting `max-server-connections` in the `config.toml` file. If too large concurrency leads to an increase of response time, it is recommended to increase the capacity by adding TiDB nodes.
 
 #### How to view the creation time of a table?
 
@@ -1036,7 +1036,7 @@ From the above results, you can get that the `add index` operation is being proc
 
 #### Does TiDB support CBO (Cost-Based Optimization)? If yes, to what extent?
 
-Yes. TiDB uses the cost-based optimizer. The cost model and statistics are constantly optimized. TiDB also supports correlation algorithms like hash join and soft merge.
+Yes. TiDB uses the cost-based optimizer. The cost model and statistics are constantly optimized. TiDB also supports join algorithms like hash join and sort-merge join.
 
 #### How to determine whether I need to execute `analyze` on a table?